In a typical drilling operation, a drill bit is rotated while being advanced into a formation within the earth. There are several types of drill bits, including roller cone bits, hammer bits and drag bits. There are many kinds of drag bits with various configurations of bit bodies, blades and cutters.
Drag bits typically include a body with a plurality of blades extending from the body with a face at a front end and a mounting pin at a rear end. The bit can be made of steel alloy, a tungsten matrix or other material. Drag bits typically have no moving parts and are formed as a single-piece body with cutting elements brazed or attached into the blades of the body. Such bits are commonly manufactured by milling a billet or sintering a powder matrix in a mold. Each blade supports a singular or a plurality of discrete cutters on the leading edge of the blades that contact, shear, grind and/or crush the rock formation in the borehole as the bit rotates to advance the borehole.
The drill string and the bit rotate about a longitudinal axis and the cutters mounted on the blades sweep a radial path in the borehole to fail rock. Cutters can be made from any durable material, but are conventionally formed from a tungsten carbide backing piece, or substrate, with a front facing table comprised of a diamond or other suitable material. The tungsten carbide substrates are formed of cemented tungsten carbide comprised of tungsten carbide particles dispersed in a cobalt binder matrix.
FIG. 1 is a schematic representation of a drilling operation 2. In conventional drilling operations a drill bit 10 is mounted on the lower end of a drill string 6 comprising drill pipe and drill collars. The drill string may be several miles long and the bit is rotated in the borehole 4 either by a motor proximate to the bit or by rotating the drill string, or both simultaneously. A pump 8 circulates drilling fluid through the drill pipe and out of the drill bit to flush rock cuttings from the bit and move them back up the annulus of the borehole. The drill string comprises sections of pipe that are threaded together at their ends to create a pipe of sufficient length to reach the bottom of the borehole 4.
Steel bits are generally machined from a single billet to produce a bit with a body and blades. Recesses to receive cutters are machined into the blades and often require special machining steps and techniques to reach parts of the blades that are obstructed by adjacent blades. A plenum is machined into the rear of the bit. The plenum is drilled with a single point tool and widened by boring. Boring is used to achieve greater accuracy in the diameter of a hole, and can be used to cut a tapered hole or enlarge a portion of a hole. Boring uses a boring tool that includes a long bar used to position a single-point tool for boring operations.
With the plenum created, the ducts are drilled from the outside face of the bit to the plenum. Drilling fluid pumped down the drill string flows through the plenum and ducts to the face of the bit to flush away cut material. An open and unrestricted duct inlet in the plenum limits turbulence or cavitation in the fluid flow as it enters the duct. Bit configurations are typically limited to including only ducts with inlets positioned near the center of the plenum on account of the difficulty under current manufacturing processes of forming ducts with expanded inlet portions for the desired flow patterns in the ducts. Accordingly, the use of ducts in other locations (e.g., near corners or walls of the plenum) is generally avoided regardless of their desirability to the performance of the bit. Machining surface features in the plenum to accommodate special duct configurations add significant cost to the bit.
An improved ferrous drill bit and a manufacturing method for ferrous bits that is less complex and costly, involves fewer steps, and encompasses a broader range of options for duct configurations would be advantageous.